1. Field of the Invention
The present invention relates to registration and density control in an image forming apparatus.
2. Description of the Related Art
Along with the development of a computer network technique, an image forming apparatus such as a printer serving as an image output terminal has rapidly become widespread. In recent years, along with the development of a technique of outputting color images, a demand for improved stability of the quality of an image forming apparatus has increased. Especially for the accuracy of superimposing the color (color registration) and the reproducibility of the density of a printed image, high stability is required despite a change in installation environment, a change with time, or differences between individual apparatuses. Since, however, color registration and an image density in an image forming apparatus vary due to a change caused by continuous use of each driving member or image generation member, a change in temperature within the apparatus, or the like, it is impossible to satisfy such a high requirement with the initial settings. To satisfy such a requirement, the image forming apparatus generally performs calibration to appropriately maintain the color registration and image density. Note that the calibration includes color registration; that is, registration control of correcting the relative position of an image of each color, and density control of correcting an image density.
In calibration, a toner image for test (to be referred to as a detection pattern hereinafter) is formed on a circulating moving member such as a photosensitive member, an intermediate transfer member, or a transfer conveyance belt, thereby measuring the position and density of the detection pattern. Based on the measurement result and the conditions under which the detection pattern has been formed, conditions for changing the color registration and image density such as a latent image writing position, an image forming magnification, a charging voltage, a developing voltage, and an exposure amount are controlled so that the color registration and image density in actual printing become appropriate.
It is impossible to perform any print operation during calibration. If, therefore, calibration is started, a user who wants to perform printing has to wait for completion of the calibration. The time taken for the calibration is, thus, desirably shorter. Japanese Patent Laid-Open No. 2001-166553 proposes a technique of shortening the calibration time by parallelly or sequentially performing registration control and density control. Furthermore, Japanese Patent Laid-Open No. 2003-186278 discloses an arrangement for detecting a detection pattern for registration control and that for density control using three or more sensors.
To perform calibration, a detection pattern may be formed on an image carrier a plurality of times in order to avoid the influence at a position on the image carrier where the detection pattern is formed. At this time, the distance between the detection patterns is set to satisfy predetermined conditions for cancelling the influence at the arrangement positions. That is, there are constraints on the arrangement positions of the detection patterns, and thus the distance between the detection patterns may have to be widened by arranging them to satisfy the constraints. This results in an increase in total length of the detection patterns, thereby prolonging the time taken to form the detection patterns and remove them thereafter.